A window shade drive possessing these class characteristics has been known from WO 91/03619. The brake disks in this prior-art arrangement are pressed against each other by a coil spring acting axially in relation to the axis of the winding shaft in order to generate the frictional forces, which are greater than the weight of the window shade unwound nearly completely. To overcome the friction, an axial cam is provided between a drive wheel and the winding shaft. Through this correspondingly strong axial forces are to be applied to overcome the spring tension and these forces become noticeable in an undesired manner during the operation of the window shade.
U.S. Pat. No. 4,433,765 discloses a window shade drive, in which a plurality of radially tensioning springs are arranged between two coaxially mounted, cylindrical elements and they automatically bring about a coupling between a drive member and a stationary component, so that the coupling can be released by rotating a drive member.